There has been conventionally known a method of forging a bevel gear, as shown in FIGS. 6A to 6D (see JP2004-58120A).
In this method, as shown in FIG. 6A, a cylindrical blank W100 is firstly prepared. When the cylindrical blank W100 is pressed in a cavity space of a die, as shown in FIG. 6, there are formed: a preliminary first body part 103a on an axially one-end side, the preliminary first body part 103a having a preliminary one-end recessed part 102 that is axially recessed in an axis-center part; and a preliminary second body part 103b on an axially other-end side, the preliminary second body part 103b projecting continuously from the axially other-end side of the preliminary first body part 103a, and the preliminary second body part 103b having a diameter smaller than that of the preliminary first body part 103a. Thus, a first intermediate article W101 is formed (first forging step).
When the first intermediate article W101 is pressed in a cavity space of another die, which is different from the die used in the first forging step, as shown in FIG. 6C, there are formed: a first body part 110a on the axially one-end side, the first body part 110a having a tooth part 109 on a radially outer circumference thereof; a second body part 110b on an axially intermediate portion, the second body part 110b projecting continuously from the axially other-end side of the first body part 110a, and having a diameter smaller than that of the first body part 110a; and a projecting part 115 on the axially other-end side, the projecting part 115 projecting continuously from a rear surface part 113 on the axially other-end side of the second body part 110b, and having a diameter smaller than that of the second body part 110b. At the same time, there is formed a one-end recessed part 107 that is axially recessed in the axis-center part on the axially one-end side of a body part 110 which is composed of the first body part 110a and the second body part 110b. Thus, a second intermediate article W102 is formed (second forging step). The tooth part 109 has a plurality of tooth crests 109a that define a tooth-crest surface which is tapered from the axially other-end side toward the axially one-end side. In addition, the second body part 110b is provided with a spline on an outer circumference thereof.
In the second forging step of this method, in order to promote plastic deformation (flow) of the blank, in a region corresponding to an outer circumference of the first body part 110a of the second intermediate article W102, the die has a portion whose dimensions are larger than the dimensions of the outer circumference of the first body part 110a. Namely, as a flow way for the blank, there is formed a superfluous space between the die and the outer circumference of the first body part 110a of the second intermediate article W102. This is based on the following view. That is, by positively promoting a partial flow of the blank, it is intended to promote the entire deformation of the blank into the second intermediate article W102.
However, because of the provision of this superfluous space, when the second intermediate article W102 is completed, there is formed, in addition to the tooth part 109, an unnecessary burr part 114 that radially projects outward from the outer circumference of the first body part 110a of the second intermediate article W102.
After the second forging step, as shown in FIG. 6D, an axis-center part 116 extending from a bottom surface of the one-side recessed part 107 to the projecting part 113 of the second intermediate article W102 is pierced and removed. However, formed by this piercing process is a bevel gear W103 which has a through hole 117 formed in the axis-center part and also still has the burr part 114. Namely, there is required a step in which the burr part 114 is removed.
In addition, since the outer circumferential surface of the preliminary first body part 103a of the first intermediate article W101, which is to be deformed into the tooth part 109, is in parallel with the axis line, a flow amount of the blank that is deformed into the tooth part 109 in the second forging step may be too much, resulting in undesired wrinkles (grooves) in the formed tooth crests 109a. 
In addition, there has been conventionally known another method of forging a bevel gear, as shown in FIGS. 7A to 7D (see, JP Patent Publication No. 3690780).
Also in this method, as shown in FIG. 7A, a cylindrical blank W200 is firstly prepared. When the cylindrical blank W200 is pressed in a cavity (diemilled) space of a die, as shown in FIG. 7B, there are formed: a preliminary first body part 203a on an axially one-end side, the preliminary first body part 203a including a tapered part 204 whose diameter is increased from the axially one-end side toward an axially other-end side; and a preliminary second body part 203b on the axially other-end side, the preliminary second body part 203b projecting continuously from the axially other-end side of the preliminary first body part 203a, and having a diameter smaller than that of the preliminary first body part 203a. At the same time, there is formed a preliminary other-end recessed part 206 that is axially recessed in an axis-center part on the axially other-end side of the preliminary second body part 203b. Thus, a first intermediate article W201 is formed (first forging step).
When the first intermediate article W201 is pressed in a cavity space of another die that is different from the die used in the first forging step, as show in FIG. 7C, there are formed; a first body part 210a on the axially one-end side, the first body part 210a having a tooth part 209 on a radially outer circumference thereof; and a second body part 210a on the axially other-end side, the second body part 210a projecting continuously from the axially other-end side of the first body part 210a, and having a diameter smaller than that of the first body part 210a. At the same time, there is formed an other-end recessed part 214 that is axially recessed in the axis-center part on the axially other-end side of a body part 210 which is composed of the first body part 210a and the second body part 210b. Thus, a second intermediate article W202 is formed (second forging step). The tooth part 209 has a plurality of tooth crests 209a that define a tooth-crest surface which is tapered from the axially other-end side to the axially one-end side.
In the second forging step of this method, in order to prevent generation of underfills in the tooth part 209, when the second intermediate article W202 is completed in the second forging step, the hermetically sealed cavity space, which is defined in the die, is designed to be completely filled with the second intermediate article W202, that is, the overall outer surface of the second intermediate article W202 is brought into contact with the surface of the die defining the cavity space. This is based on the following view. Namely, in order to prevent generation of underfills, a high pressure applied by the cavity space is transmitted to the entire blank as if to envelope the same, while the blank is deformed into the second intermediate article W202.
After the second forging step of this method, as shown in FIG. 7D, an axis-center part 216 extending from the one end of the second intermediate article W202 to a bottom surface of the other-end recessed part 214 is pierced. Then, there is obtained a bevel gear W203 which has a through hole 217 formed in the axis-center part. In this method, a step of removing a burr part is unnecessary. The bevel gear W203 can be generally used as a bevel gear for a side gear for a differential gear.
However, in this method, when the second intermediate article W202 is completed, in the hermetically sealed cavity space, since the overall outer surface of the second intermediate article W202 is brought into contact with the surface of the die defining the cavity space (i.e., since there is no “clearance” therebetween), a large stress may be applied to the die. Thus, there is a problem in that a life time of the die is short (the die tends to be broken within a shorter period of time).
In addition, there is described a forging method on page 292 in “Plastic Process Guide” issued by Corona Publishing Co., Ltd. In this method, as shown in FIG. 8A, a cylindrical blank W300 is prepared. Then, by pressing the cylindrical blank W300 in a cavity space of a predetermined die, there is formed a first intermediate article W301 having a cross-sectional shape as shown in FIG. 8B. Then, by using another die, as shown in FIG. 8C, there is formed a second intermediate article W302 having a tooth part 309 on a radially outer circumference thereof. Thereafter, as shown in FIG. 8D, there is formed a third intermediate article W303 having a burr part 314 remaining on an outer circumference thereof. After that, an axis-center part is pierced, and the burr part 314 is removed. Thus, as shown in FIG. 8E, a bevel gear W305 having a through hole 317 is formed.
Similarly to the forging method that has been described with reference to FIGS. 6A to 6D, this forging method requires a step of removing the burr part 314.
There is also described another forging method on page 292 in the “Plastic Process Guide” issued by Corona Publishing Co., Ltd. In this method, as shown in FIG. 9A, a cylindrical blank W400 is prepared. Then, by pressing the cylindrical blank W400 in a cavity space of a predetermined die, there is formed a first intermediate article W401 having a cross-sectional shape as shown in FIG. 9B. Then, by using another die, as shown in FIG. 9C, a second intermediate article W403 having a tooth part 409 on a radially outer circumference thereof is formed in a hermetically sealed cavity space. Thereafter, an axis-center part is pierced. Thus, as show in FIG. 9D, a bevel gear W404 having a through hole 417 is formed.
Similarly to the forging method that has been described with reference to FIGS. 7A to 7D, this forging method has a problem in that, since a large stress is applied to the die for obtaining the second intermediate article W403, a life time of the die is short.